【二叉树】对称二叉树

class Solution:
    def isSymmetrical(self, pRoot):
        if pRoot == None:
            return True
        return self.helper(pRoot, pRoot)
    def helper(self, p1, p2):
        if p1 == None and p2 == None:
            return True
        if p1 == None or p2 == None:
            return False
        if p1.val != p2.val:
            return False
        else:
            return self.helper(p1.left,p2.right) and self.helper(p1.right,p2.left)